1. Field of Invention
This invention is related to interferometric optical devices of wavefront division, particularly to interferometric optical devices used in topography, biomedical tomography, and optical data storage technologies.
2. Description of Prior Art
An interferometric optical profiler is a device which uses optical interference to measure topography of a sample surface. Topography measurements are often required in semiconductor, data storage, and fiberoptic telecommunication industries. For example, inspection of a silicon wafer includes surface measurements in semiconductor manufacturing. There are two major types of interferometric optical profilers: imaging and scanning types. Both types rely on interference of amplitude division.
An imaging type profiler produces an optical image of a surface area. It is usually based on Michelson, Mirau, Linnik, or Fizeau interferometers, which use a beam splitter to split a beam from a light source into two beams by amplitude division. The two beams travel along separate paths and are reflected by a reference surface and a sample surface, respectively. The reflected beams are then recombined by the beam splitter to construct an interference pattern. The interference pattern depends upon two factors: optical path length difference between the two paths and the sample's surface profile. The measurement is fast, but is sensitive to vibration since vibration changes the optical path length difference.
A scanning type profiler scans a surface to collect topography data. It is usually based on a concentric-beam interferometer or a common-path polarization interferometer. Although a scanning type profiler creates two beams by a beam splitter as an imaging type does, the two beams either travel on same optical path, or have side-by-side paths. As a result, vibration effects are reduced. Due to its scanning nature, a scanning type profiler is limited in measurement speed.
Imaging and scanning type profilers have quite different structures. It is difficult to integrate the two into one setup to save cost. Therefore both profilers are needed for circumstances requiring a fast profile survey and less vibration sensitivity.
Accordingly, current interferometric optical profilers based on interference of amplitude-division suffer from difficulties of integrating imaging and scanning type profilers into one arrangement.
Optical coherence tomography (OCT) is an imaging technology capable of measuring three-dimensional structures of highly scattering media, such as a variety of biological tissues. It has great potentials in biomedical applications. An OCT system employs a low-coherence light source which emits a beam with a relatively short coherence length. Currently at the heart of OCT is an amplitude division interferometer, usually a Michelson interferometer. Like the above discussed interferometers used for an optical profiler, an OCT system splits a beam into two beams by a beam splitter. One beam propagates to a reference surface along a reference optical path, and the other beam to a sample medium along a sample optical path. The beams reflected by the reference surface and the sample medium are then recombined by the beam splitter.
Due to the nature of low coherence, the combined beams interfere with each other only when their optical path length difference is within the beam coherence length. The interference intensity and pattern contrast reach a maximum when the two optical path lengths are matched. For highly scattering sample media, various sample paths yield different optical path lengths, depending upon where a beam is reflected inside the media. Since a reference optical path length can be adjusted to match a sample optical path length, tuning the reference path length results in interference between the reference beam and a sample beam which is reflected from a layer at a depth inside the media. The interference intensity and patterns are related to the layer's optical properties, such as refractive index, birefringence, scattering coefficient, etc. A beam coherence length determines measurement resolution along the beam propagation direction. The shorter the coherence length is, the higher the measurement resolution. By combing the low coherence interference technique with a laterally scanning mechanism, a three-dimensional image can be constructed.
Since the reference and sample optical paths are separate, they might experience different environmental changes. Thus the optical path length difference is sensitive to environmental variations, so does the interferometric measurement. The setup is also bulky due to the two separate paths.
Accordingly, the current OCT system based on interference of amplitude-division suffers from sensitivity to environmental variations and a bulky structure.
Besides biomedical applications, the OCT technology also has advantages in multi-layer optical data storage. Multi-layer optical storage media, which contain a three-dimensional distribution of reflectors, resemble highly scattering media, and can be detected by an OCT system. But the current OCT measurement is sensitive to sample vibration due to two separate optical paths, and it is difficult to apply the technology to read out data on a rotating optical disc.
Multi-layer optical storage media have been proposed with OCT readout methods. See, for example, U.S. Pat. No. 5,883,875 (1999) to Coufal, et al. and U.S. Pat. No. 6,072,765 (2000) to Rolland, et al. As a result, the multi-layer media only contain storage layers and not a reference reflector. The reflector is usually integrated with the OCT optical structure. Since readout results depend upon an optical path length to a storage layer, any vibration causes a change of the optical path length and brings measurement errors.
Accordingly, a multi-layer optical data storage device has difficulties in utilizing current OCT technology, and readout results of current multi-layer optical storage media are sensitive to vibration.
Like above discussions, most interferometric devices in use today are amplitude-division types. However, wavefront division interferometers are also known. For example, Bhagavatula describes an interferometric filter containing a spatial phase modulator in U.S. Pat. No. 5,841,583 (1998). The spatial phase modulator divides a beam into two portions by wavefront-division and creates a phase difference between them. The two portions have side-by-side optical paths. Interference of the portions occurs without generating two separate optical paths in different directions. An interferometric optical device which uses wavefront division is simpler and more compact than one that uses amplitude division, thanks to its optical path feature. However, the interferometric filter is mainly for light wave processing. Its ability to interact with surrounding media is limited besides detecting a refractive index along its beam path.